Well if uncertain about the fluid level after the transmission was installed. Just pull the fill plug and let it drain down. Or add fluid until it reaches the plug level.

Additional drag just might make it harder to shift cleanly if really overfull. Also I see no advantage to being overfilled in general. Certainly under filled is especially not a good scenario.

As a general expectation weakness in the syncro ring between first and second is probably the most common problem with these particular transmissions. They were a heavy duty transmission for the loading they see in our diesels by original design. Capable of handling far more power than our cars produce. Mercedes also put them behind some pretty powerful gas engines in the day.

Most of the older manual transmissions where fairly easy to deal with. Or at least in my experience they were. The last one I did was a first to second syncro ring in our large truck.

Again a simple job basically. Although you wanted a heavy transmission jack for that one.

Then again I am getting pretty old. Few things remain fairly simple and straightforward today in comparison. To what they were. Has to be viewed as a relative understatement.